1. Technical Field of the Invention
The present invention relates to optical network elements and, more particularly, to a fiber Bragg grating filter (FBGF) having a wide tuning range and a method of making the same using Shape Memory Alloys.
2. Description of Related Art
As networks face increasing bandwidth demand and diminishing fiber availability in the existing fiber plant, network providers are migrating towards a new network technology called the optical network. Optical networks are high-capacity telecommunications networks comprised of optical and opto-electronic technologies and components, and provide wavelength-based services in addition to signal routing, grooming, and restoration at the wavelength level. These networks, based on the emergence of the so-called optical layer operating entirely in the optical domain in transport networks, can not only support extraordinary capacity (up to terabits per second (Tbps)), but also provide reduced costs for bandwidth-intensive applications such as the Internet, interactive video-on-demand and multimedia, and advanced digital services.
Of the several key enabling technologies necessary for the successful deployment of optical networks, two are particularly significant: dense wavelength division multiplexing (DWDM) and Erbium-Doped Fiber Amplifiers (EDFAs). DWDM is a fiber-optic transmission technique that has emerged as a crucial component for facilitating the transmission of diverse payloads regardless of their bit-rate and format over the optical layer DWDM increases the capacity of embedded fiber by first assigning incoming optical signals to specific wavelengths within a designated frequency band (i.e., channels separated by sub-nanometer spacing) and then multiplexing the resulting signals out onto a single fiber. Because incoming signals are not terminated in the optical layer, the interface is bit-rate and format independent, allowing service/network providers to integrate the DWDM technology with existing equipment in the network.
By combining multiple optical signals using DWDM, they can be amplified as a group and transported over a single fiber to increase capacity in a cost-effective manner. Each signal carried can be at a different rate (e.g., Optical Carrier (OC)-3, OC-12, OC-48, etc.) and in a different format (e.g., Synchronous Optical Network (SONET) and its companion Synchronous Digital Hierarchy (SDH), Asynchronous Transfer Mode (ATM), Internet Protocol (IP) data, etc.).
Current advances in DWDM technologies allow a large number of wavelengths to be multiplexed over a fiber using sub-nanometer spacing. For example, up to 32 channels or carriers may be spaced 100 GHz apart (equal to 0.8 nm) in a multiplexed optical signal operating at around 1550 nm. In contrast, some of the standardized, xe2x80x9ccoarsexe2x80x9d wavelength separations include 200 GHz spacing (1.6 nm) and 400 GHz spacing (3.2 nm), both at around 1550 nm.
Several advances are also taking place in the field of optical amplifiers which operate in a specific band of frequency spectrum and boost lightwave signals to extend their reach without converting them back to electrical form. To optically amplify the individual wavelengths of multiplexed signals, optical amplifiers need to have a gain bandpass that extends over the entire range of the DWDM signal""s bandwidth. For example, for 32 channels with a spacing of 0.8 nm around the 1500 nm band, the signal bandwidth is about 26 nm and, accordingly, the spectral gain profile of the optical amplifier should cover at least this range. Advanced optical amplifiers such as the EDFAsxe2x80x94which have a gain profile of about 30 to 50 nmxe2x80x94are currently being employed in optical networks using DWDM transmission techniques.
Those skilled in the art should readily recognize that in order to fully realize the benefits of such advances as DWDM techniques and EDFAs in optical networks, the ability to separate the individual wavelengths in a multiplexed optical signal is critical because these wavelengths typically need to be routed to individual detectors at the end of the transmission. Although various optical filtering technologies are currently available for this purpose, there exist several drawbacks and deficiencies in the state-of-the-art solutions.
For example, wavelength separators using interference filters and Fabry-Perot filters typically have a low resolution which renders them a poor choice for the sub-nanometer spacing of the current DWDM techniques. Further, these filters do not have a quick enough response time for achieving any degree of tunability, that is, the ability to select different wavelengths using the same filter, in a practical manner.
Optical filters made of fiber Bragg gratings offer excellent resolution characteristics. However, current implementations such as, for example, acoustic-optical fiber Bragg gratings and piezo-electric Bragg gratings, allow tuning over a few nanometers only, which approximates to about 5 or 6 channels. Clearly, this tuning range is insufficient to cover the channel bandwidth of the advanced DWDM systems described hereinabove.
Based upon the foregoing, it should be apparent that there is an acute need for an optical filter solution that provides a wide tuning range for selecting wavelengths among a large number of channels available in today""s DWDM systems. Additionally, it would be advantageous to have a narrow optical passband (for the selected wavelength) so as to be able to tune to a particular wavelength more precisely without optical crosstalk effects. It would be of further advantage to provide the capability for tuning over a range that is at least co-extensive with the gain profiles of the advanced EDFAs used in current optical networks. The present invention provides such a solution.
Accordingly, the present invention is directed in one aspect to a tunable optical filter having a wide response range. The optical filter comprises a fiber having a selected length for conducting optical signals, wherein a Bragg grating with a predetermined period of a refractive index (i.e., the grating pitch) is included for reflecting back a reflected optical signal having the Bragg resonance wavelength. An actuator is coupled to the fiber for varying the period of the refractive index by changing the length of the fiber. A closed-loop controller is included for controlling the actuator by monitoring a relevant optical or electrical parameter (or, a plurality of parameters) associated with the reflected optical signal.
In one exemplary embodiment, the actuator comprises a plurality of Shape Memory Alloy (SMA) members having substantially the same length and diameter as the fiber member. In a presently preferred implementation, six SMA members are disposed around the fiber member in a hexagonal configuration (xe2x80x9chex-packxe2x80x9d) such that the ends of the SMA members are bonded to the ends of the fiber member. A current or thermal source is also included as part of the actuator arrangement to provide a controlled energy input (current or heat) to the SMA members which compress upon receiving the energy. Because of the contraction in the length of the SMA members, the length of the fiber member is also compressed. Accordingly, the grating pitch of the fiber Bragg grating member is shortened as well. The Bragg resonance wavelengthxe2x80x94which matches the wavelength of the reflected optical signalxe2x80x94tunes to the shorter end of the tuning range in correspondence to the change in the grating pitch.
In another aspect, the present invention is directed to a method of making a wide tuning range optical filter using a fiber Bragg grating member of a predetermined grating pitch. Preferably, a single-mode fiber of selected length and diameter is exposed to coherent UV radiation. An interference pattern of the UV radiation is thereby generated along the length of the single-mode fiber, wherein the interference fringe pattern""s periodicity matches the desired grating pitch. The interaction between the UV light and the fiber produces periodic changes in the index of refraction of the fiber such that the periodic changes spatially correspond to the interference fringe pattern""s periodicity.
Thereafter, multiple SMA members having substantially the same length and diameter as the fiber (with the Bragg grating xe2x80x9cwrittenxe2x80x9d into it as set forth above) are disposed longitudinally around the fiber such that the ends of the SMA members are bonded to the ends of the fiber. In a preferred exemplary embodiment of the present invention, six SMA members are arranged in a hex-pack configuration surrounding the fiber. A current or thermal source is coupled to the SMA members to provide energy thereto. A closed-loop controller is connected to the current/thermal source to modulate the amount of energy supplied to the SMA members. By controlling the amount of energy, the Bragg grating pitch is varied so that the reflected optical signals tune to appropriate wavelengths.